Hamm et al., 2021 reported that voluntary wheel running (R) was complementary to micro-dystrophin gene therapy (GT) in mdx mice, a model of Duchenne muscular dystrophy (DMD). After 21 weeks of running, time to fatigue on a treadmill for the mdxRGT mice was increased 1.8-fold compared to mdxGT mice (no run) and ~5-fold compared to mdx mice (no micro-dystrophin, no run). Fatigue times for mdxRGT were similar to wild type runners (WTR), while mdxGT and WT (no run) were also similar. The diaphragm is an important muscle for endurance exercise. Remarkably, diaphragm power in mdxRGT was depressed compared to mdxGT, suggesting a negative impact of running on GT. To explore mechanisms to explain this decrease, transcriptome profiles for each of the study groups were assessed. RNASeq data revealed differentially expressed genes (DEGs) from groupwise comparisons. Transcripts identified using the Jackson Labs' Gene Expression Database and extensive literature review were organized into a master signaling pathway composed of two sub-pathways: muscle regeneration and fast-slow fiber type shift. Both sub-pathways were hypothesized to explain the improved treadmill performance despite decreased diaphragm power in mdxRGT as potential adaptive mechanisms. Analysis revealed that GT alone (mdxGT) rescued transcriptome expression to WT values in the mdx phenotype more than GT and running combined (mdxRGT). This outcome indicates that, at the 26-week timepoint of sacrifice, the signaling of the transcripts in the muscle regeneration and fast-slow fiber type shift sub-pathways was likely not responsible for the observed improved running performance of mdxRGT compared to mdx. / Master of Science / Muscular dystrophy is a group of diseases characterized by progressive muscle wasting and loss of function. Duchenne muscular dystrophy (DMD) is the most common of these conditions, with an occurrence of 18 per 100,000 live births. The muscles of people with DMD lack a protein called dystrophin, which provides structural integrity for muscle fibers during contraction. This lack of dystrophin leads to muscle deterioration over time, leading to people with DMD typically being wheelchair-bound by ten years of age. Animal models of DMD have been created over time to help study this condition. One such model, the mdx mouse, was used in the study that led to this thesis project. In this study, Hamm et al., 2021, some of these mdx mice were given a micro-dystrophin gene therapy (GT). This GT aimed to deliver a smaller, but still functional version of the missing dystrophin protein to the mice, which has been shown to be beneficial in other studies. This study aimed to measure the effect of this GT when combined with voluntary wheel running. As people with DMD cannot exercise under current clinical guidelines, measuring the response of mdx mice to GT and running combined is an important step in determining the safety of such a treatment in human patients. In the study conducted by Hamm et al., 2021, the mice that received the micro-dystrophin GT and access to running wheels (mdxRGT) performed almost twice as well on a treadmill running test than the mice that received GT alone (mdxGT). Despite this positive result, the mdxRGT mice showed decreased diaphragm power generation compared to mdxGT mice. As the diaphragm is the most important breathing muscle, it is also very important for running performance; therefore, the decreased diaphragm power generation seen in mdxRGT mice is apparently contrary to their improved running performance. To explain this discrepancy, this thesis project examined the diaphragm transcriptome of the different groups in Hamm et al., 2021. The transcriptome is the sum total of messenger ribonucleic acid (mRNA) expressed in a given tissue. Deoxyribonucleic acid, or DNA, is transcribed into mRNA, which is then translated into protein. As such, this thesis project looked at the mRNA expressed in the diaphragm of the mice in the various groups of Hamm et al., 2021, specifically comparing the mdxGT and mdxRGT groups. Important mRNA transcripts, or genes, were identified and assembled into signaling pathways, cascades that highlight how transcripts affect each other and ultimately lead to function once they are translated into their corresponding proteins. Two such signaling pathways were generated based on mechanisms that were thought to contribute to the improved running performance-decreased diaphragm power discrepancy in mdxRGT mice -slow fiber type shifting, and muscle fiber regeneration. The expression of many of the mRNA transcripts in these resulting pathways was closer to the control group in mdxGT compared to mdxRGT. The control group was made up of healthy mice, and as such, their transcript expression level is seen as normal. This outcome of mdxGT having more similar expression to the control group than mdxRGT suggests that the expression of the transcripts included in the two signaling pathways (fast-slow fiber type shift and muscle fiber regeneration) likely did not explain the improved running performance despite decreased diaphragm power in mdxRGT mice. As such, future studies are warranted.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/108382 |
| Date | 16 February 2022 |
| Creators | McQueen, Lucas Flynn |
| Contributors | Human Nutrition, Foods and Exercise, Grange, Robert W., Mack, David L., Schmelz, Eva Maria, Drake, Joshua Chadwick |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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